1. Field of the Invention
The invention relates to a reticle or photomask used in photolithography, and more particularly to a reticle which is capable of forming a rectangular mask pattern with high accuracy.
2. Description of the Related Art
As a semiconductor device has been required to have a higher integration and a smaller size, a pattern to be formed on semiconductor device is also required to be reduced in a size.
In a method of fabricating a semiconductor device, photolithography steps are generally carried out. In order to increase a throughput and enable a semiconductor device to have an increased capacity, a unit for exposing a pattern to a light is required to expose a pattern to a light in a wide area at a time.
As a result, a minification ratio in the unit is reduced down to 4 from 5. In addition, a pattern is formed in a smaller size than before. Accordingly, it is necessary to reduce a size of a mask pattern formed on a reticle, resulting in an increase in difficulty in fabrication of a reticle.
There is an increased need for a highly accurate reticle. In particular, edges of a pattern or corners of a minute pattern in a reticle are likely to be rounded. Rounded edges or corners of a pattern would deteriorate an accuracy at which a pattern is formed on a semiconductor device, and further deteriorate a fabrication yield of a semiconductor device.
Specifically, since a pattern is formed on a reticle by etching a light-impermeable film formed on a transparent substrate by photolithography, it would be quite difficult to form a desired pattern with high accuracy, as a size of a pattern is reduced. In particular, sharpness in edges and/or corners is likely to be degraded due to diffraction and/or interference when a pattern is exposed to a light, resulting in that edges and/or corners of a pattern are rounded.
As a solution to such a problem, Japanese Unexamined Patent Publication No. 6-118624 has suggested a method of exposing a pattern to a light. In the suggested method, there are used a first reticle including a pattern extending in an X-axis direction, and a second reticle including a pattern extending in a Y-axis direction. A pattern is exposed twice to a light through the use of the first and second reticles. As a result, a pattern with high accuracy can be formed at an area having been exposed twice to a light.
However, the suggested method is accompanied with problems that a step of exposing a pattern to a light has to be carried out twice, and that it is necessary to make registration between the first and second reticles in the two steps of exposing a pattern to a light. If the registration is not made properly, it would be impossible to have a desired pattern with high accuracy.
Japanese Unexamined Patent Publication No. 60-7431 has suggested a reticle including a transparent plate, a first thin film pattern composed of opaque material and formed partially on a surface of the transparent plate, a transparent spacer formed on the surface of the transparent plate such that the spacer covers at least a part of the first thin film pattern therewith, and a second thin film pattern composed of opaque material and formed partially on the spacer.
Japanese Unexamined Patent Publication No. 62-45026 has suggested a method of fabricating a semiconductor integrated circuit, including the steps of exposing a resist to a light through a first mask having a pattern extending in a first direction, and exposing the resist to a light through a second make having a pattern extending in a second direction perpendicular to the first direction.
Japanese Patent Publication No. 4-56477 has suggested a method of fabricating a Josephson junction device including a lower electrode, an insulating film formed on the lower electrode and formed with a hole, a barrier layer formed above the lower electrode, and an upper electrode formed above the insulating film and the barrier layer. The method includes the steps of forming a first insulating film on the lower electrode through a first line resist pattern, removing the first line resist pattern to form a first hole in the form of a line, forming a second insulating film through a second line resist pattern intersecting with the first hole, and removing the second line resist pattern to thereby form a hole for making Josephson junction.
Japanese Unexamined Patent Publication No. 7-106764 has suggested a ceramic multi-layered circuit board including a plurality of first line patterns each comprised of broken lines with a space therebetween and extending in a X-axis direction, and a plurality of second line patterns each comprised of broken lines with a space and extending in a Y-axis direction. The first and second line patterns are arranged on a ceramic insulating layer in matrix such that the space defines a cross-point. The first and second line patterns sandwiching the cross-point therebetween are electrically connected to each other through a first hole formed through the ceramic insulating layer at the cross-point. The first hole does not reach a bottom surface of the ceramic insulating layer. The above-mentioned first and second line patterns are electrically connected to first and second patterns formed on a surface of a lower ceramic insulating layer, through a via-hole formed throughout the ceramic insulating layer at the cross-point.
The problem that a rectangular pattern cannot be formed with high accuracy remains unsolved even in the above-mentioned Publications.
In view of the above-mentioned problems, it is an object of the present invention to provide a reticle which can expose a rectangular pattern to a light with high accuracy without increasing complexity in a step of exposing a pattern to a light.
There is provided a reticle including (a) a first substrate including a first light-permeable substrate, and a first pattern formed on the first light-permeable substrate and having a first light transmittance, and (b) a second substrate including a second light-permeable substrate, and a second pattern formed on the second light-permeable substrate and having a second light transmittance, the first and second substrates being coupled to each other such that the first and second patterns face each other, a part of the first and second patterns at which the first and second patterns overlap each other, defining a light-impermeable pattern.
It is preferable that the first and second patterns are composed of the same material.
For instance, the light-impermeable pattern may be a rectangular pattern.
It is preferable that the first and second light transmittances are equal to or smaller than 0.7.
It is preferable that the reticle further includes a light-permeable adhesive layer to be sandwiched between the first and second substrates for coupling the first and second substrates together, in which case, the light-permeable adhesive layer is formed preferably in an area at which the first and second patterns do not overlap each other.
There is further provided a reticle including (a) a first substrate including a first light-permeable substrate formed at a surface thereof with first recesses in a pattern, and a first pattern formed in the first recesses and having a first light transmittance, and (b) a second substrate including a second light-permeable substrate formed at a surface thereof with second recesses in a pattern, and a second pattern formed in the second recesses and having a second light transmittance, the first and second substrates being coupled to each other such that the first and second patterns face each other, a part of the first and second patterns at which the first and second patterns overlap each other, defining a light-impermeable pattern.
There is still further provided a reticle including (a) a first substrate including a first light-permeable substrate, and a first pattern formed on the first light-permeable substrate and having a first light transmittance, and (b) a second substrate including a second light-permeable substrate formed at a surface thereof with second recesses in a pattern, and a second pattern formed in the second recesses and having a second light transmittance, the first and second substrates being coupled to each other such that the first and second patterns face each other, a part of the first and second patterns at which the first and second patterns overlap each other, defining a light-impermeable pattern.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, the reticle includes a light-impermeable pattern defined by an area in which the first and second patterns overlap each other. In particular, corners of a rectangular pattern are defined by intersection of a side of the first pattern with a side of the second pattern. As a result, it would be possible to have a reticle presenting a highly accurate rectangular pattern unlike a conventional reticle which presents a rectangular pattern having rounded corners.
In addition, a rectangular pattern could be exposed to a light through one reticle in only one exposure step. This means that the number of steps of exposing a pattern to a light is not increased in comparison with a conventional method, and that it is no longer necessary to make registration between two reticles. Hence, it is ensured to simplify the exposure step and avoid reduction in an accuracy of a rectangular pattern which reduction is caused by error in registration of two reticles.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.